board: bsh: imx8mn-bsh-smm-s2: fix U-Boot startup

Commit e3329a1e ("configs/imx8mn_bsh_smm_s2[_pro]: bump Linux to
6.12.23 and U-Boot to 2025.04") broke the U-Boot boot process on the
BSH SMM S2 board. I didn’t notice it because I mistakenly tested the
patch on the SMM S2PRO board, which boots from eMMC instead of NAND.
Next time, I won’t forget to test on both boards :).

Fixes: e3329a1e ("configs/imx8mn_bsh_smm_s2[_pro]: bump Linux to 6.12.23 and U-Boot to 2025.04")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>